Question

Can someone help me solve this conversion?

Answer 1

For every 1 liter of ghee, there are 905 grams. For every 1 liter of butter, there are 911 grams. The liter is the same, so that's your unit of volume. For every n units of volume, that ratio stays the same. ie, it's just 905g/911b. If you want to change that,  then you'd have 905g/911b = xg/yb. For every x value, there's a unique y value, and vice versa.

Answer 2

Practically, those values are so close that you might as well replace them 1:1. Any errors introduced by measurement, small amounts of butter/ghee left on utensils, product variation, local weather, etc. will be larger.